The PAC is polyaluminum chloride while the PAS is polyaluminum sulfate.  PAC chemistries and performance are influenced by the chemistry and the process used to produce the product.  Thus, there is no "better" chemistry, PAC or PAS.  Testing is needed to determine the best product for the type of water being treated.  

Also, the basicity of the PAC or PAS will impact performance and various products produced under various production processes (even raw material can influence the changes) will not be the same in basicity.  Thus, water treatment performance differences can be measured between seemingly identical products.